Gemara

Once, a man was eyeing a woman, and he became sick from desire for her. Doctors said that his only cure is if he will have relations with her.

Rabanan: It is forbidden, even if he will die.

Doctors: Let her stand naked in front of him (perhaps this will help)!

Rabanan: It is forbidden, even if he will die.

R. Yakov bar Idi or R. Shmuel bar Nachmani said that she was married. The other said that she was Penuyah (single); nevertheless, Chachamim were stringent because it would have been embarrassing to her family, or to preserve the Kedushah of Benos Yisrael.

Avodah Zarah 20a (Beraisa): R. Gamliel was in Har ha'Bayis, and he saw a beautiful Nochris. He said "Mah Rabu Ma'asecha Hash-m."

Question (Beraisa): "V'Nishmarta mi'Kol Davar Ra" - one may not look at a beautiful woman, even a Penuyah, or at a married woman, even if she is ugly. One may not look at a woman's colored clothing.

Answer: He saw her (suddenly) when he turned a corner.

Shabbos 64a (Tana d'vei R. Yishmael): The soldiers who defeated Midyan needed a Korban to atone for pleasure from seeing forbidden women.

(Rav Sheshes): The Torah lists external Tachshitim (e.g. rings) with inner Tachshitim (girdles) to teach that one who looks (for pleasure) at a woman's pinky needs atonement like one who looks at her Ervah.

Berachos 24a: If two are sleeping on a bed, each may turn his head away to say Keri'as Shema, even if their buttocks touch the other!

This supports Rav Huna, who says that buttocks are not Ervah.

Support #2 (Mishnah): A woman can sit naked and take Chalah (with a blessing), for she can cover her Ervah (even though her buttocks are exposed).

Rejection: The case is, her buttocks are recessed in the ground.

(R. Yitzchak): An exposed (square) Tefach of part of a woman's body (that should be covered) is Ervah.

He need not forbid looking at it. Even gazing even at a woman's finger is like gazing at her Ervah! Rather, one may not say Keri'as Shema facing his wife if a Tefach is exposed.

(Rav Chisda): A woman's thigh is considered Ervah.

(Rav Sheshes): A woman's hair is considered Ervah.

Rishonim

Rambam (Hilchos Isurei Bi'ah 21:2): One may not gaze at the beauty of a (woman forbidden due to) Ervah. One who does is suspected of Arayos. If he does so for pleasure, he is lashed mid'Rabanan. Looking even at the pinky for pleasure is like looking at the place of Ervah.

Rambam (ibid. 21): One may not look at women hanging up laundry. It is even forbidden to look at colored clothing of a woman he recognizes lest he come to have thoughts.

Ba'al ha'Ma'or (18a): One may not look at a naked woman, even in a case of mortal danger. If she is married, this is letter of the law. One may transgress Arayos when the Me'anes (one who threatens to kill him if he does not consent) seeks his own pleasure.

Rebuttal (Milchamos Hash-m): Chachamim did not allow the girl to help cure the man, even though he intended for his own benefit!

Hagahas ha'Bach (10): The Ba'al ha'Ma'or forbids when one wants to benefit from the Aveirah itself. Also Chidushei ha'Ran (Pesachim 25b DH v'Hevi'u and Nimukei Yosef (Bava Kama 43b) distinguish like this.

Suggestion: One must forfeit his life rather than benefit from Ervah, e.g. to go to a beach where women are not covered properly.

Rebuttal (Igros Moshe EH 1:56): This is unlike the Ba'al ha'Ma'or's case. There, he wanted to benefit from Ervah to cure his lust for sin. This is worse than one who seeks to save himself from a Me'anes. One may not look at her naked to cure himself, even though it is not a concrete Isur, but if he was threatened 'look at her or be killed', perhaps it is permitted, since he need not touch her. The Ramban could admit, for it is not a concrete Isur. Without intent for pleasure, it is a light Isur; even touching, hugging and kissing have no Lav or lashes then Shach (YD 157:11). Also, the Isur Arayos of looking is like the case in Sanhedrin, when she is brought for him to look at her naked. When he goes to where women are naked, this is a different Isur of bringing himself to Hirhur, which could lead to a seminal emission. (One must forfeit his life only for Arayos.)

Rashba (Berachos 24a DH v'Ha d'Omar): The Rif did not mention an exposed Tefach, thigh, or hair (regarding Kri'as Shema). The Ra'avad says that the Rif holds that since we said that buttocks are not Ervah, all the more so these. The Ra'avad himself says that we are not concerned for buttocks because they do not distract a man.

Beis Yosef (OC 75 DH Kosav ha'Rashba): The Rambam does not mention these regarding Kri'as Shema. It seems that he holds that they are forbidden to hear or see; the Isur was not said regarding Shema. L'Halachah, we hold like the Rambam, but l'Chatchilah one should be careful at the time of Shema.

Yere'im (45): Chachamim warned not to look at Arayos; an Asmachta is "guard yourself from every evil matter." The soldiers needed Kaparah for Hirhurim.

Yad Ramah (Bava Basra 164:52): The Aveirah of Hirhurim is even concerning a Penuyah.

Kolbo (64 DH ha'Megale'ach): One who transgressed Arayos or spilled his seed must be extra careful to avoid anything that causes Hirhurim, e.g. looking at a women's face, or the areas near the Ervah or breasts, or her clothing even on the wall, or have any thoughts of women other than his wife.

Poskim

Shulchan Aruch (EH 21:1): One must distance himself very very much from women. One may not look at an Ervah's beauty. Looking even at the pinky for pleasure is like looking at the place of Ervah. It is forbidden even to see her hair. One who does any of these is lashed mid'Rabanan.

Beis Yosef (DH Kasuv b'Orchos, citing R. Yonah): The Torah forbids looking at a married woman - "v'Lo Sasuru... v'Acharei Eineichem". Kesuvim forbid looking at a Penuyah - "Bris Korati l'Einai u'Mah Esbonen Al Besulah." One who withholds his eyes from looking will merit to behold the Shechinah, Midah k'Neged Midah - "v'Otzem Einav me'R'os b'Ra; ...Melech b'Yafyo Techezenah Einecha" (Vayikra Rabah 23:13). The Torah forbids thinking even about a Penuyah - "V'Nishmarta mi'Kol Davar Ra" - do not have thoughts during the day, for they lead to Tum'ah at night.

Beis Shmuel (2): The Rambam forbids looking at a married woman only mid'Rabanan. All agree that looking at a Penuyah is forbidden from Kesuvim, and that the Torah forbids Hirhur even about a Penuyah.

Shulchan Aruch (ibid. 4): One may look at Ishto (his wife) Nidah, even if it is pleasurable. Since she will be permitted later, he will not sin.

Source (Beis Yosef DH v'Chen): Shabbos 64b permits a Nidah to adorn himself lest she become repulsive to her husband. We infer that he may look at her.

Shulchan Aruch (OC 75:1): One who faces an exposed (square) Tefach (in a place that is normally covered) of a woman may not say Shma.

Question: At all times it is forbidden to look even at the pinky!

Answer (Bach DH Tefach): At other times it is forbidden only if he intends to look, for then he gets pleasure.

Question: This implies that one may see areas normally exposed without intent for pleasure. If so, why didn't the Gemara answer that R. Gamliel did not intend for pleasure?

Answer (Igros Moshe OC 1:40): R. Gamliel blessed 'how great are Your works', i.e. he got pleasure. Therefore, we needed to answer that he turned a corner and had no time to close his eyes. One must always strive to avoid looking at women, lest it be pleasurable. When walking in the street one should try to look down unless this would make him prone to injury.

Shulchan Aruch (YD 195:7): When one's wife is Nidah, he may not look at her heel or parts that are normally covered. He may look at places that are normally exposed even if it is pleasurable.